This film's antagonist, Guiron (named for "guillotine"), is a giant frog-like monster with a knife on his head, which is 100 times harder than diamond. Along the knife is a hole that shoots small magnetic shuriken. He can also jump really high.
Two boys, Akio and Tom, spy in their telescope a spaceship that descends into a nearby field. With Akio's little sister in town, they bicycle out the next day to investigate. The boys enter the spaceship and accidentally take off to an alien planet named Terra, while Tomoko is left behind, unable to convince any adults (except kindly but eccentric police officer Kondo) of the boys' misadventure. On Terra, the boys encounter two beautiful but sinister Terran women and the kaiju Guiron, who kills Space Gyaos. Gamera eventually tracks the boys to Terra and kills Guiron. The Terran women are killed and Gamera, Akio, and Tom go back to Earth.
- Space Gyaos wasn't going to be in this film; a new kaiju was going to be in it, but due to time and budget limitations, the Gyaos props from Gamera vs. Gyaos were simply painted silver.
- In the Alpha Video U.S. release, the scene where Space Gyaos was killed by Guiron was removed for its graphic content.